The present invention involves the use of a multi-stage membrane system for gas, vapor, and liquid separations. This membrane system can reduce inter-stage compression cost, increase product recovery and product purity for gas, vapor, and liquid separations. In this configuration, two types of membranes will be used. One is a membrane with both high selectivity and high permeance or at least high selectivity, which is at least greater than 20, preferably greater than 30, but the cost of manufacturing such a membrane is relatively high. The other membrane is the commercially available membrane with lower selectivity.
Multi-stage membranes are used to increase the product recovery or the product purity for commercial application. For example, in a conventional two-stage membrane system to recover the useful product, a permeate that passes through the primary membrane will be compressed and then will pass through the secondary membrane. The residue from the secondary membrane is then recycled to the primary membrane. This two-stage membrane configuration can significantly increase the product recovery compared to a one-stage membrane system. The two-stage system can also be used to obtain high purity permeate product. However, the required compressor power is generally high since the permeate from the primary membrane is compressed in this process configuration.
It is important to reduce the compression cost while maintaining the level of product recovery in any new configuration. In the present invention, a preliminary membrane (referred to herein as a “pre-membrane”) with both high selectivity and high permeance or at least one with high selectivity, which is at least greater than 20, preferably greater than 30, is used. The permeate from the pre-membrane will not be compressed and sent to the secondary membrane, but instead, it will be directly sent to either the waste stream or product stream. Since the selectivity of the pre-membrane unit is very high, the membrane will provide either low product (as retentate) loss, or high purity product (as permeate).
It is further desirable to increase the retentate product recovery and/or permeate product purity by applying the high selectivity and high permeance or at least high selectivity, which is at least greater than 20, preferably greater than 30, membrane as both the pre-membrane and the secondary membrane in the proposed membrane system configuration. Since the pre-membrane area and the second-stage membrane area are relatively small compared to that of the primary membrane, the new membrane system will not significantly increase the cost of whole system even the high cost membranes are used for the pre-membrane and secondary membrane.
Preferably the membrane materials for the pre-membrane and secondary membrane of the multi-stage membrane system in the present invention are selected from polybenzoxazole polymers and cross-linked polybenzoxazole polymers that have both high selectivity and high permeability or have at least high selectivity.
A recent publication in the journal SCIENCE reported on a new type of high permeability polybenzoxazole polymer membrane for gas separations (Ho Bum Park et al, SCIENCE 318, 254 (2007)). These polybenzoxazole membranes are prepared from high temperature thermal rearrangement of polyimide polymer membranes containing pendent hydroxyl groups ortho to the heterocyclic imide nitrogen. These polybenzoxazole polymer membranes exhibited extremely high CO2 permeability (>1000 Barrer) for CO2/CH4 separation. These polybenzoxazole polymer membranes have very good thermal stability at elevated temperature.